Patent GB-1395872-A (GB)
Title: Compatible split band distance measuring method and apparatus
IPC: G01S13/86
Filed: 1973-12-14
Abstract: 1395872 Radar TULL AVIATION CORP 14 Dec 1973 [26 Dec 1972] 58135/73 Heading H4D Distance measuring apparatus comprises airborne equipment, Fig. 6, including a transmitter 62 operable to transmit interrogation signals in a first frequency band, a first receiver 64 for receiving and processing response signals from a ground station, Fig. 7, which are in that frequency band, whereby to determine distance. The airborne equipment also includes a second receiver 40 for receiving response signals which are in a second frequency band, frequency conversion means 44 giving an output in the first frequency band, and means connecting said output to the first receiver 64 for determination of the distance. Means are provided to prevent application to the first receiver of response signals which have not been converted in frequency to the first frequency band, when the second receiver is in operation. As described, the transmitter 62 and the first receiver 64 both operate within the L-band of frequencies (e.g. from 1À0 to 1À2 GHz), and the second receiver 40 operates within the C-band of frequencies (e.g. from 5À0 to 5À25 GHz). A ranging circuit 48 sends a pulse to a line 50 to initiate interrogation. The line 50 is coupled directly to one input of an OR circuit 54 and indirectly, through a delay line 52A, to the other input thereof. The delay line 52A has two selectable taps 58, 100 the former of which provides a standard delay of [twelve microseconds for use when a response within the L-band of frequencies is required from a ground station, and the latter which provides a different delay, of say eighteen microseconds, when a C-band response is required. The pulse pair from the OR circuit 54 operates a modulator 60 for the transmitter 62, and two RF bursts are generated which are coupled through an input-output connection 14 and a directional coupler 18C to an aerial 20, whence they are radiated. At the ground station the pulses are received on an aerial 22 and are demodulated by a receiver 26, after which they are coupled through a delay device 28 to two pulse spacing recognition devices 30, 30A. The delay device 28 is to enable the provision of a standard fifty microsecond delay between receipt of an interrogation signal at a ground station and emission of a response signal therefrom. The devices 30, 30A each feature an AND gate 37, 37A with a first input connected directly to the delay device 28 and with a second input connected thereto through a delay line 31, 31A of respective delays twelve and eighteen microseconds. Thus according to the spacing between the pulse-pair from the airborne transmitter 62, either the device 30 causes an L-band transmitter 104 to respond via an aerial 22 or the device 30A causes a C-band transmitter 34 to respond via an aerial 36. Adjacent channel L-band transmitters are not activated when C-band responses are required, because the interrogating L-band pulse pair spacing is not twelve microseconds. When L- band response signals are sent they are received on the aerial 20 and are directly coupled to the receiver 64. This gives an output to the ranging circuit 48, which determines the time interval between interrogation and response, and transmits a distance value signal to an indicating device 12. When C-band response signals are sent they are received on an aerial 38 and are coupled to the receiver 40. This receiver includes means which converts the received signals to conventional ILS instrument landing frequencies, and a receiver 42 may be provided for use therewith. The frequency converter 44 completes frequency conversion to the L-band and the signals are then coupled via the directional coupler 18A to the receiver 64. In other embodiments the standard twelve microsecond interrogation pulse spacing is used whether L-band or C-band responses are required, but in the latter case the aerial 20 is inhibited during reception to avoid unwanted L-band responses. As shown in Fig. 5, the input-output connection 14 may be connected through a directional coupler 86 and a delay line 94 to two PIN switches 82,84. The switch 82 is normally closed, so that connection is made through a line 46 to the frequency converter 44 and the receiver 40, for utilization of C-band responses, and the switch 84 is normally open, so disconnecting the aerial 20. Upon operation of the transmitter 62 a signal from the directional coupler 86 is detected and amplified, and operates a monostable multi-vibrator circuit 90. This provides a switching signal to the PIN switches whilst the interrogating pulse pair is passing, so that the line 46 is disconnected and the aerial 20 is connected. The switches 82, 84 may be held in their operated states by means of a switch 16A, when reception of L-band responses is required. In another embodiment the aerial 20 is directly connected by means of switches 16-1, 16-2, Fig. 3, to the input-output connection 14 for reception of L-band responses, but is connected by the switch 16-1 to one part of a duplexing device, e.g. a circulator 72, when C-band responses are to be received, the input-output connection 14 being connected by the switch 16-2 to another part of said circulator. The line 46 from the frequency converter 44 and the receiver 40 is connected to another part of the circulator, and a matched load 76 to another. Any L-band responses received on the aerial 20 when the switches 16-1, 16-2 are set for the reception of C-band responses, are absorbed in the load 76. In another embodiment, further attenuation of unwanted L-band responses is obtained by using two circulators in series, each with a matched absorbing load (Fig. 4, not shown).